1. Field of the Invention
The present invention relates to a surge current suppressing circuit, and more particularly to a surge current suppressing circuit using an air-core inductor to suppress an inrush current.
2. Description of Related Art
With reference to FIG. 5, a traditional approach for anti-lightning mainly relies on adding a metal-oxide-varistor (MOV) 50 after an input fuse 51. During normal operations, the MOV 50 presents itself as a high impedance and is just like an open-circuit, isolated from and not influencing the circuit operation. As lightning surge strikes and comes into the circuit from the AC power lines, the MOV 50 will be activated by this high surge voltage and turned into low impedance, shunting and absorbing high energy to itself to protect a following circuit.
Though featuring with high energy absorbing capacity and quick reaction time, the MOV 50 has a service-life problem. The characteristics of the MOV 50 will deteriorate with the number of times of activation, for example, activation voltage will become lower and lower and leakage current will gradually increase. When the activation voltage decreases to be close to a normal operating voltage of the circuit, the MOV 50 may be quite often activated by the normal operating voltage and eventually becomes overheated and damaged, or even gets on fire. Thus in application it is sometimes needed to place a thermal fuse 52 to work together with MOV 50, wherein the thermal fuse 52 will be activated first and isolate the MOV 50 before the MOV 50 gets overheated.
As described above, there will be an instantaneous high impulse current flowing into the MOV 50 when the lightning surge strikes. The big current will also pass through the input fuse 51. In order to cope with this high impulse current, the input fuse 51 needs to have high “melting integral” (I2t) to withstand the large current and not to be burned out. However, high melting integral usually also means high current rating, thus resulting in using a fuse with much higher current rating than needed. To cope with this, sometimes an extra fuse 53 with adequate current rating will be put after the MOV 50 to protect the circuit.
Furthermore, safety rules require a “single-fault test.” During the short-circuit test, there will be a very high current flowing into the fuse. To avoid arcing or getting on fire, a fuse with high breaking capacity has to be used. This kind of high breaking capacity fuse has bigger size and higher cost, moreover, to have the same melting integral level, high breaking capacity fuses need to be made with much higher current rating than the normal ones.